The response of mesophyll conductance to short-term CO₂ variation is related to stomatal conductance

The response of mesophyll conductance (g_m) to CO₂ plays a key role in photosynthesis and ecosystem carbon cycles under climate change. Despite numerous studies, there is still debate about how g_m responds to short-term CO₂ variations. Here we used multiple methods and looked at the relationship between stomatal conductance to CO₂ (g_sc) and g_m to address this aspect. We measured chlorophyll fluorescence parameters and online carbon isotope discrimination (Δ) at different CO₂ mole fractions in sunflower (Helianthus annuus L.), cowpea (Vigna unguiculata L.), and wheat (Triticum aestivum L.) leaves. The variable J and Δ based methods showed that g_m decreased with an increase in CO₂ mole fraction, and so did stomatal conductance. There were linear relationships between g_m and g_sc across CO₂ mole fractions. g_m obtained from A-C_i curve fitting method was higher than that from the variable J method and was not representative of g_m under the growth CO₂ concentration. g_m could be estimated by empirical models analogous to the Ball-Berry model and the USO model for stomatal conductance. Our results suggest that g_m and g_sc respond in a coordinated manner to short-term variations in CO₂, providing new insight into the role of g_m in photosynthesis modelling.